In the fabrication of light-emitting diodes, group III-V compound semiconductors, such as GaP, GaAsP, GaInP, AlGaAs, AlGaP and AlGaInP, are common. Typically, a material of a growth substrate of a conventional light-emitting diode adopts N-type gallium arsenide (GaAs). The growth substrate composed of N-type GaAs can absorb light, so that most of the photons produced by the active layer of the light-emitting diode while being emitted towards the growth substrate are absorbed by the growth substrate, thus seriously affecting the light emitting efficiency of the light-emitting diode device.
In order to prevent light being absorbed by the substrate, a method that directly bonds the GaAs light-emitting diode wafer to the silicon (Si) substrate after the GaAs light-emitting diode wafer is stripped off the GaAs substrate has been developed. Additionally, the U.S. Pat. No. 5,376,580 (application date: Mar. 19, 1993) filed by Hewlett-Packard Co., U.S.A. disclosed a technology about directly bonding the AlGaAs light-emitting diode wafer to the other substrate after the AlGaAs light-emitting diode wafer is stripped off the GaAs substrate. However, the disadvantages of the U.S. Pat. No. 5,376,580 include processing difficulties and low yields caused by the need to consider the consistency of the lattice direction between the bonding wafers, since the bonding mediums are semiconductors.
In the conventional bonding process, a bonding step has to be performed first, and processes of an illuminant epitaxial structure and a permanent substrate are performed, so that the bonding temperature is limited to a value larger than the process temperature of the illuminant epitaxial structure. Under higher bonding temperature, the material of an adhesive layer must adopt materials with higher melting points and larger hardness, so that degradation in the operating of the light-emitting diode easily occurs.
Further, in order to improve the current-spreading effect of a light-emitting diode, a typical design is to increase the area of electrodes. However, the electrodes are opaque, so that increasing of the area of the electrodes results in increasing the opaque area, thereby decreasing brightness of the light-emitting diode.